Technological development and increased demand for mobile equipment have led to a rapid increase in the demand for secondary batteries as an energy source. Among other things, a great deal of research and study has been focused on lithium secondary batteries having high energy density and voltage. These lithium secondary batteries are also commercially available and widely used.
In general, the lithium secondary battery is comprised of a cathode, an anode and a separator therebetween, with addition of an electrolyte. In this connection, the electrolyte is used in the form of a material which contains a suitable amount of a lithium salt dissolved in an organic solvent. Examples of the lithium salt added to the electrolyte may include conventionally used materials such as LiPF6, LiBF4, LiClO4, LiN(C2F5SO3)2, and the like. These materials serve as a lithium ion source in the battery to enable the basic operation of the lithium battery.
Currently available electrolytes undergo various side reactions during charge/discharge processes, and the thus-produced by-products may be responsible for deterioration of the battery performance.
Particularly when a lithium salt LiPF6 is incorporated into the electrolyte, LiPF6 should be present in ionic forms of Li+ and PF6−. Usually, contrary to intentions, side reactions take place with production of an unstable by-product PF5, which subsequently reacts with H2O to result in formation of HF. HF causes destruction of the SEI layer and cathode dissolution, which becomes more severe at high temperatures.
Upon initial charge of the lithium secondary battery, lithium ions deintercalate from the cathode and intercalate between layers of the graphite electrode used as the anode. At this time, the lithium ions react with carbon atoms of the anode to form a passivation film, called Solid Electrolyte Interface (SEI), on the anode surface. Once the SEI layer is formed, the lithium ions do not undergo the side reaction with the graphite anode or other materials. Therefore, destruction of the SEI layer by HF resulting from the side reaction of LiPF6 in the electrolyte may result in serious malfunction of the battery operation.
In order to prevent the problems as mentioned above, additives may be used in the electrolyte. A primary function of conventional electrolyte additives was to prevent formation of by-products occurring upon charge and discharge of the battery.
A conventional lithium secondary battery undergoes numerous side reactions as well as a favorable forward reaction for the lithium salt LiPF6, so the operation efficiency of the battery is lowered. Major side reactions may include formation of LiF and PF5 from decomposition of LiPF6 (side reaction-1), formation of HF and POF3 from reaction of PF5 (from side reaction-1) with trace water in the electrolyte (side reaction-2), and HF-induced SEI destruction on the anode (side reaction-3).
Furthermore, additional materials other than HF may be produced such as HCl, HBr, and HI, depending upon kinds of lithium salts used as the electrolyte. These by-products may act as acid thereby exerting potentially deleterious functions as HF does.
In this connection, Korean Patent Application Publication No. 2006-92074 A1, assigned to the present applicant, proposes a technique of improving the high-temperature storage performance of a battery via addition of an ammonium compound to a non-aqueous electrolyte. The inventors of the present invention have made extensive investigations on such a subject to improve the high-temperature storage performance of the battery. As a result, they have found that when a certain compound of the present invention among these ammonium compounds is used in the fabrication of the battery, solubility of that compound in the electrolyte is significantly increased to thereby greatly improve the high-temperature storage performance to a degree that cannot be achieved by the above Korean Patent. This fact can be further confirmed in Examples and Comparative Examples which will be illustrated hereinafter. Meanwhile, even though it is unrelated to the present invention, U.S. Pat. No. 4,535,389 discloses a technique of improving high-temperature output characteristics via addition of ammonium benzoate to a capacitor electrolyte. However, the above prior art is a technique which applies to the capacitor. Further, ammonium benzoate, as described above, has low solubility in an electrolyte for the secondary battery, and as such it is impossible to improve the high-temperature storage performance to a desired degree.
Upon considering the fact that performance of a secondary battery at high temperatures becomes more important, there is an urgent need for development of a more effective additive.